Double U-Tube Geothermal Borehole Operation Under Phase Change Conditions

RÉSUMÉ Les systèmes de pompe à chaleurs couplées à des échangeurs géothermiques verticaux constituent des alternatives intéressantes pour la climatisation/chauffage des espaces et le chauffage de l'eau chaude en raison de leur haute efficacité et de leur faible impact sur l'environnement. Cependant, les coûts de forage demeurent un obstacle à leur utilisation généralisée. Les coûts de forage sont directement reliés à la profondeur des puits. Dans le cas d'installations avec puits unique, la profondeur dépend principalement de l'extraction maximale de la chaleur du sol pendant le pic des charges de chauffage du bâtiment. Pour diminuer l’extraction maximale de chaleur du sol, il a été suggéré de recharger le sol en utilisant l'énergie solaire. Cependant, dans les installations à simple puits, l'injection de chaleur solaire ne réduit pas la longueur de forage de façon significative puisque l'injection solaire ne coïncide pas nécessairement avec le pic de charge du bâtiment. Par ailleurs, la chaleur solaire injectée se dissipe dans le sol sans faire augmenter la température du sol à proximité du puits de façon significative. Dans ce projet, une nouvelle alternative est proposée pour réduire la longueur de forage dans les installations avec puits unique. Il s'agit d'une pompe à chaleur géothermique couplée avec un système solaire dont l'échangeur géothermique est constitué d’un puits à deux tubes en U avec deux circuits indépendants entouré par un anneau de sable saturé. Cette configuration est utilisée pour l'extraction de la chaleur dans un circuit, combiné avec une pompe à chaleur, et l'injection thermique dans l'autre circuit en utilisant l'énergie solaire. Dans cette configuration, l'échangeur géothermique agit comme un échangeur de chaleur entre le capteur solaire et la pompe à chaleur. Lors des périodes de pointe de chauffage du bâtiment, généralement la nuit lorsque l'énergie solaire n'est pas disponible, la pompe à chaleur extrait l'énergie du sol et, dans certains cas, le sable saturé gèle en formant un anneau autour du puits. Cela a pour effet de ralentir la baisse de la température de retour à la pompe à chaleur et tire parti de la teneur en énergie relativement élevée associée à la chaleur latente de fusion de l'eau dans le sable. Lorsque l'énergie solaire est disponible, la chaleur solaire est injectée dans le second tube en U pour faire fondre l'anneau de glace. Pour évaluer précisément les conséquences de l'utilisation du système proposé, des modèles théoriques pour les puits et le sol sont développés dans cette étude.----------ABSTRACT Conventional ground coupled heat pump systems with vertical ground heat exchangers constitute attractive alternatives for space conditioning and domestic hot water heating due to their high efficiency and environmental friendliness. However, borehole costs remain a barrier for their widespread utilization. Borehole costs are mainly driven by borehole depths which, in single borehole installations, are mainly dependent on peak ground load heat extraction during peak building heating loads. To mitigate the peak ground heat removal, it has been suggested to recharge the ground using solar energy. However, in single borehole installations, solar heat injection does not reduce the borehole length significantly since solar energy injection is not necessarily coincident with the peak building load. Furthermore, the injected solar heat dissipates into the ground without making notable increases on ground temperature near the borehole. In this work, a new solar assisted ground coupled heat pump alternative is proposed to reduce the borehole length in single borehole installations. The system under study consists of a double Utube borehole with two independent circuits surrounded by a saturated sand ring. This configuration is used for heat extraction in one circuit, combined with a heat pump, and simultaneous thermal recharging in the other circuit using solar energy. In effect, it acts as a heat exchanger between the solar thermal collector and the heat pump. During peak building loads, usually at night when solar energy is unavailable, the heat pump extracts energy from the ground and in some cases the saturated sand freezes. This slows down the decrease in the return temperature to the heat pump and takes advantage of the relatively high energy content associated with the latent heat of fusion of water in the sand. When solar energy is available, solar heat is injected in the second U-tube to melt the frozen saturated ring. To evaluate precisely the expected consequences of using the proposed system, theoretical models for the borehole and the ground are developed in this study. The borehole model accounts for the double U-tube with two independent circuits and the numerical ground model can handle freezing and thawing in the saturated region in the immediate vicinity of the borehole as well as pure conduction heat transfer in the ground

IMPACT OF GREY WATER HEAT RECOVERY ON THE ELECTRICAL DEMAND OF DOMESTIC HOT WATER HEATERS

ABSTRACT Grey water heat exchangers (GWHE) are used to recuperate part of the energy contained in grey waters. The configuration used in this study recuperates part of the energy contained in the grey water from showers to pre-heat domestic hot water. Previous simulations studies have shown that this configuration can recuperate part of the energy that would otherwise be lost and allow the use of smaller electric domestic hot water (DHW) tanks. This paper focuses on the impact that GWHE have on peak electrical demand from electric DHW tanks. Simulations are performed using TRNSYS with a standard DHW tank model and a special GWHE model. A total of ten different yearly water draw profiles are statistically generated at 1 minute intervals. This small time step is required in order to capture the transient effects in the GWHE. It is shown that the aggregated effect of these profiles corresponds to the electrical consumption measurements performed on 600 residential electric DHW tanks. Simulation results show that GWHE have an impact on the peak electrical demand with reductions of 119.4 Watts (10.4% reduction) at 8:00 and 184.0 Watts (21.5% reduction) at 22:00. On an annual basis, the energy required for DHW heating is 4501 and 5299 kW-hr with and without a GWHE, respectively

Technical and economic evaluation of a ground source heat pump with thermal and battery energy storage systems for residential dwellings in Quebec

Ground source heat pump (GSHP) systems have been identified as promising solutions to help buildings achieve future carbon neutral targets. However, two main barriers decelerate the wide adoption of this technology in every location. One barrier is the high initial cost, which is sometimes justified by high local energy prices. The other potential barrier that always imposes a major bottleneck on a transition to electrification by using GSHP systems is the impact on the electrical grid at peak demand times. In this study, a solution has been proposed to address both issues by integrating thermal and battery energy storage systems into a conventional GSHP system. Numerical simulations are performed to evaluate the technical and economic feasibility of the proposed configuration. In addition, a comparative analysis is performed with other commonly recognized GSHP configurations with and without thermal energy storage. Results show that the new configuration requires a 22% shorter borehole by using the thermal energy storage, and reduces the annual peak electricity demand by almost 500kW cumulatively by using the battery energy storage. These benefits contribute to improving the net present value of the new configuration by 23% compared to the conventional GSHP configuration

Heat pump capacity effects on peak electricity consumption and total length of self- and solar-assisted shallow ground heat exchanger networks

A new "self-assisted" Ground Source Heat Pump (GSHP) system configuration is proposed to address the relatively high peak electricity demand of undersized GSHP systems equipped with auxiliary electric heater. In this configuration, ground heat exchangers (GHE) have two independent circuits: the first circuit is used to inject the extra heat produced by the heat pump into the ground during off-peak operations, while the second circuit is used to extract heat in the winter and reject heat in the summer for space heating and cooling, respectively. This configuration is compared against a "solar-assisted" configuration and a conventional single U-tube configuration. An analytical model for shallow GHE networks is used to evaluate the effects of the heat pump nominal capacity and the borehole total length on the total electricity consumption and peak electricity demand of the three configurations. Results show that the self-assisted configuration reduces the peak electricity demand by 47%, in a case with a 29% undersized GHE network and a 16% undersized heat pump nominal capacity, while it increases the total energy consumption by 4.1%. Using a solar-assisted configuration for the same sizing parameters reduces the peak electricity demand by only 6.3% and the total energy consumption by 3.8%

Calibration of thermal response test (TRT) units with a virtual borehole

This paper presents the development of a virtual borehole (VB) used to calibrate the ground thermal conductivity obtained from thermal response test (TRT) units. The VB is composed of an aboveground plate heat exchanger and chiller unit carefully controlled to mimic the thermal behavior of the ground by reproducing the time evolution of the mean fluid temperature for a user-selected ground thermal conductivity. During calibration, TRT units are connected to the VB just like if they were connected to a real borehole. The various components of the VB are described including the characterization of the heat exchanger, the implementation of a resistance-capacitance (RC) borehole model, and the required control algorithm. The VB concept is successfully tested by comparing the results obtained on a real borehole to those given by the VB for given conditions. An uncertainty analysis reveals that the ground thermal conductivity set by the VB is accurate to within ± 2.5%. The usefulness of the VB is then demonstrated by calibrating a commercially available TRT unit for two ground thermal conductivities, 1.0 and 3.0 W m-1 K -1. Results of this calibration indicate that the TRT unit evaluates ground thermal conductivities of 1.02 and 3.18 W m-1 k-1, respectively

Direct expansion ground source heat pump using carbon dioxide as refrigerant: Test facility and theoretical model presentation

In an attempt to address recent challenges on using natural refrigerants and to develop further knowledge and expertise in the field of direct expansion ground source heat pump (DX-GSHP), an experimental transcritical carbon dioxide (CO2) test bench was built at CanmetENERGY Research Laboratory. A previously developed theoretical model of the system was modified and validated against a set of experimental results and adopted to investigate the system performance in a wide operating range. A parametric analysis was also performed using the theoretical model for understanding the system and at exploring the performance improvement actions for future installations. Validation results showed that the model predicts the experimentation very well within the uncertainty of the measurement. Furthermore, parametric analysis showed that improper control of some parameters such as gas cooler CO2 outlet temperature and discharge compressor pressure can degrade the system performance by up to 25% and the heat pump heating capacity by 7.5%

Virtual borehole for thermal response test unit calibration: Test facility and concept development

Precise calculation of the borehole length requires good estimation of the ground thermal conductivity. In practice, the ground thermal conductivity is measured in-situ at a specific location using what is referred to as a thermal response test (TRT) unit. This paper presents a novel virtual borehole (VB) concept for calibrating TRT units. The VB replaces a real borehole with an above-ground compact heat exchanger and a chiller unit to mimic the thermal behavior of the ground with a user-set virtual ground thermal conductivity. In an attempt to develop the VB concept, three control scenarios are examined to emulate the ground thermal response for different thermal conductivity values. A test bench was built at the CanmetENERGY-Varennes research laboratory to validate the VB concept experimentally. A test is performed to calibrate a commercially available TRT unit for a thermal conductivity value of 3 W m-1 K-1. The TRT unit connected to the VB reported a value of 3.18 W m-1 K-1 representing a 6% error

Economic optimization and parametric analysis of large hybrid ground source heat pump systems: A case study

Hybrid ground source heat pump systems offer a solution to reduce initial costs and make systems more economically viable. Their design is however complex and their financial profitability difficult to establish. The design of hybrid system is usually determined by following rough rules and is neither mathematically rigorous nor optimized. In this paper, a methodology recently introduced by the same authors for economic optimization of hybrid ground source heat pump systems is used to carry out a parametric analysis and assess the impact of uncertainty on the optimal design solution. The results show that all the parameters have significant impact on the optimization, and the ground heat exchanger construction costs and ground source heat pump COP had the most impact on the net present value. However trends are difficult to observe because if the non-linear nature of the problem, and thus there is a need for more robust optimization of hybrid GSHP systems under uncertainty

Carbon dioxide evaporation process in direct expansion geothermal boreholes

Ground Heat Exchangers (GHE) play an important role in the performance of Ground Source Heat Pumps (GSHP). The impact is even more significant in direct expansion GSHP (DX-GSHP) systems as the refrigerant used in the heat pump also acts as the heat transfer fluid for the GHE. In this study, several experiments were carried out to investigate the performance of GHEs in a carbon dioxide (CO2) DX-GSHP. The evaporation of CO2 in the GHE was studied under various mass flow rates and number of active boreholes. For this purpose, a transcritical CO2 DX-GSHP test facility was built and fully equipped at CanmetENERGY-Varennes research laboratory. It was found that a partial two-phase flow regime along the GHE decreases the performance compared to the full two-phase flow and it has to be avoided for more efficient DX-GSHP systems

Detailed Theoretical Characterization of a Transcritical CO2 Direct Expansion Ground Source Heat Pump Water Heater

A new avenue in modern heat pump technology is related to the use of natural refrigerants such as carbon dioxide (CO2). The use of CO2 in direct expansion ground source heat pumps (DX-GSHP) has also gained significant interest as it offers opportunities for cost reduction of the ground loop, albeit some challenges remain in their development, design and use. To address these challenges and to characterize CO2-DX-GSHP performance for water heating applications, a detailed theoretical model and a fully-instrumented test apparatus was developed and built at CanmetENERGY Research Laboratory. The theoretical model was validated against a set of experimental results and adopted to investigate the performance of the system over a wide operating range. Validation results showed that the model predicts the experimental results within the measurement uncertainty. A detailed system performance analysis was also performed using the theoretical model to understand the system behavior and explore the actions required for performance improvement in future installations. The results of the analysis showed that improper design and control of some components, such as the gas cooler and ground heat exchanger can degrade the system performance by up to 25%, and the heat pump heating capacity by 7.5%